Down-regulation of translation driven by hepatitis C virus internal ribosomal entry site by the 3′ untranslated region of RNA
The genome of hepatitis C virus (HCV) is a single-stranded RNA of positive polarity that has a poly(U/C) tract followed by a highly conserved 98-nt sequence, termed the X region, in the 3′ untranslated region (UTR). To investigate the effect of the 3′UTR on the HCV translation that depends on the internal ribosomal entry site (IRES), we prepared a deletion HCV RNA, MA▵, that lacked the RNA region from nt 1286 to 8785. A series of MA▵ RNAs that differ in the primary structure of their 3′UTR, were generated and examined for their translation efficiencies in reticulocyte lysates. Deletion of the poly(U/C) tract and/or stem-loop structure (SL) 3 region of 3′X resulted in enhancement of the translation efficiency. Translation of MA▵ RNA was inhibited by the addition of recombinant polypyrimidine tract-binding protein (PTB). A similar inhibition by PTB, however, was observed when an RNA lacking the poly(U/C) tract or SL3 region was used. The inhibitory effect by PTB was not obvious for MA▵(1041) RNA composed of nt 1 to 1041 but MA▵(8928) RNA composed of nt 1 to 1285 and nt 8786 to 8928. These results suggest that the observed down-regulation of HCV translation by the 3′UTR is mediated by some host factor(s) other than PTB, and that a PTB site for inhibition resides in the coding sequence of nt 1042 to 8928 of MA▵ RNA.
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